The present invention relates to a process for the preparation of a catalytic composition for the polymerization of alpha-olefins and to the catalytic composition thus obtained. It also relates to a process for the polymerization of alpha-olefins by means of such a catalytic composition and more particularly to a process for the polymerization of propylene and of ethylene.
It is known to polymerize, with high yields, alpha-olefins by means of a catalyst containing a transition metal compound containing at least one possibly substituted cyclopentadiene radical and an activator chosen from aluminoxanes and ionizing agents. However, such catalysts lead to the production of polymers of mediocre morphology and it proves to be absolutely essential to support them on porous particles. The supports most often used are inorganic supports such as, more particularly, silicas. However, these compounds, although they allow the constituents of the catalyst to be supported effectively, have the disadvantage of leading to the formation of polymers containing not insignificant amounts of inorganic compounds, generally called ash, which reduce the performance of the polymers and more particularly their processibility and the surface appearance of the products obtained. Furthermore, sometimes it is found that the productivity of the active species is very significantly reduced.
The use of a polymer support and more particularly of polyolefin supports allows these problems to be partially solved. This is because, since such a support is compatible with the final polymer, it results, in the end, in polymers whose ash content is particularly low (U.S. Pat. No. 5,556,893 - SOLVAY). However, since these supports are particularly inert with respect to active species, they allow only partial attachment of the latter. Consequently, the efficiency of the preparation reaction is decreased, which results in economically less profitable processes. Furthermore, during polymerization, free active species may be observed which lead to the formation of fine polymer particles which disturb the polymerization and make the final polymer more difficult to handle.
These various phenomena explain a lower efficiency of the catalyst preparation reaction which is economically prejudicial. Attempts have been made to remedy this problem by using supports having a particular porosity and by subjecting the catalyst thus obtained to a gas-phase prepolymerization (EP 598,543). However, the efficiency of the catalyst preparation reaction remains insufficient and it is difficult to avoid the formation of agglomerates and blocks during the gas-phase prepolymerization. Moreover, the prepolymerization reaction, which involves the formation of relatively small amounts of prepolymer on a particulate support is difficult to carry out in the gas phase.